One conventionally viable method of sealing together plastic-coated, fibre-based packaging laminates is so-called ultrasound sealing. In accordance with this technology, the material layers which are to be fused and sealed together are clamped between an ultrasound horn and an abutment. The ultrasound horn vibrates the material, and because of different types of hysteresis losses in the material and in the interface therebetween, the material will be heated up so that the plastic partly melts and, because of the compression, is fused and welded together. However, this technology is conventionally employed and. well-known to a person skilled in the art and will not be described in greater detail here.
EP708022B1 discloses the ultrasound sealing of the top seal on a so-called gable top package. In accordance with this construction, the material which is to be sealed is pressed against the abutment with the aid of the ultrasound horn. This publication focuses on solving a problem inherent in vibrating free edges which could give rise to dust. This is solved by means of a special arm which extends parallel with the ultrasound horn and abuts against the free edge in order thereby to reduce the vibrations in the free edge.
U.S. Pat. No. 4,581,873 discloses another construction for ultrasound sealing of the upper seal of a brick-shaped package. In this construction, the ultrasound horn is suspended in a complex linkage system which is activated in that a moment of force is applied on a pivot in one linkage. In its turn, the abutment is also suspended in a z-shaped linkage which in turn is activated in that another moment of force is applied on another pivot in this linkage. The publication focuses on realising a separate driving of the ultrasound horn and the abutment. Once the ultrasound horn and the abutment are in the correct position, a compression force is applied on them in that a compression cylinder is activated. This compression cylinder acts by the intermediary of a unit which is fixedly anchored to the ultrasound horn and a linkage which in turn acts on a heel on a unit fixedly anchored to the abutment. By such means, the ultrasound horn will be displaced in that the entire suspension apparatus for the ultrasound horn is moved towards the package and the abutment. Because the first linkage is stationary, this movement of the ultrasound horn will in turn be absorbed by a spring which, after completed sealing, will return the ultrasound horn to the position where the horn and the abutment are operated separately from each other.
WO01/094234 discloses an ultrasound horn and an abutment for sealing a tubular packaging blank with a so-called overlap seal. Such a seal is formed in that the tube is created by two longitudinal edges being sealed together in that the one edge is laid on the outside of the material adjacent the second edge. In order to be adapted for sealing a tube of such type, the ultrasound horn is provided with a recess which is adapted to accommodate the extra material thickness which occurs at the overlap seal.
WO96/21595 discloses another system where the abutment is provided with a recess for accommodating the larger material thickness at the overlap joint.
Sealing systems of the above-mentioned types are constructed usually so as to obtain a correct seal in a repeatable manner. They must be simple in their basic design and construction and be capable of sealing packages in a correct manner even if the packaging material and the configuration or position of the package vary within certain limits. Moreover, they must often be capable of pre-forming the package in connection with the sealing operation so as to realise a seal which is formed after the final folding of the package. The above-described designs and constructions are not entirely satisfactory in respect of these criteria. The first construction is adapted for a certain formation of a gable top package but not for forming a brick-shaped package with downwardly folded corner flaps. The second construction is far too complex and it is moreover difficult to obtain a correct compressive force when compressed air is to be fed to the cylinder when this is to realise the compression of the packaging material between the ultrasound horn and the abutment. Both of the latter constructions suffer from the drawback that the package and its overlap seal must be fitted into the recess in the ultrasound horn and the abutment, respectively, in order that a correct seal be obtained.
Thus, there is no satisfactory solution to the basic constructional requirements which are in place. Consequently, one object of the present invention is to realise an apparatus which satisfies, or in its basic design and construction makes it easy to satisfy the above-outlined and other constructional requirements which are placed on a sealing apparatus of the type disclosed by way of introduction.